ALL of those types of marriages are presently banned in the United States. Would it be only logical and fair to allow those as well?A brother and sister want to get married? Don't give a ****. People who don't know each other want to get married? Don't give a ****. Cheating? Polygamy? Don't give a ****. Though there are logical arguments against people of very close relation getting it on given the understanding of science and genetics. And there's arguments against polygamy given that almost every instance of polygamous society in the modern world has been one built on oppression, sexism, subjugation, and exile. Also Utah wasn't allowed into the Republic till it denounced polygamy; so there's precedent for that one.
But none of this would matter if there were no marriage license. Think the Founders had to ask government's permission to be married? Think again.